Re-export identity to allow users to use a different version of reqwest

[avandecreme]

I was trying to bump reqwest to version 0.12.x in my project.

However, I couldn't because the latest version of vaultrs uses version 0.11.x so I was not able to build an Identity:

error[E0308]: mismatched types
   |
90 |             identity,
   |             ^^^^^^^^ expected `reqwest::tls::Identity`, found `reqwest::Identity`
   |
   = note: `reqwest::Identity` and `reqwest::tls::Identity` have similar names, but are actually distinct types
note: `reqwest::Identity` is defined in crate `reqwest`
  --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/reqwest-0.12.2/src/tls.rs:77:1
   |
77 | pub struct Identity {
   | ^^^^^^^^^^^^^^^^^^^
note: `reqwest::tls::Identity` is defined in crate `reqwest`
  --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/reqwest-0.11.27/src/tls.rs:75:1
   |
75 | pub struct Identity {
   | ^^^^^^^^^^^^^^^^^^^
   = note: perhaps two different versions of crate `reqwest` are being used?

By having vaultrs re-export Identity, I can then import Identity from vaultrs::client and have the correct version.